The Treasures of Trappist-1 | Space Time

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Matt O'Dowd (0:48)

This episode is supported by LegalZoom. Last week in a NASA press release, we learned about an amazing new discovery. A nearby Red Dwarf star was discovered to have not one but seven Earth-like planets. And any of them may be capable of supporting life.

The Trappist-1 planetary system was discovered by Dr. Michal Gilon and team and reported in the journal Nature. The initial discovery of three planets in this system was made in 2015 with the Transienting Planets and Planetesimals Small Telescope, Trappist, at the La Silla Observatory in Chile. Follow-up with the Spitzer Space Telescope has now revealed an additional four planets. Both telescopes used the transit method, watching for the dimming of the central star as the planets pass in front of it. Never before have we seen so many Earth-like planets in one place. Any one of them could bear liquid water, maybe even life. The Trappist-1 system lies 39 and a half light years away in the constellation of Aquarius. The star, Trappist-1A, is an ultra-cool M dwarf star. About 10% the sun's diameter and less than 10% its mass. It's also much colder, about 2,500 Kelvin compared to the sun's 5,800 Kelvin. The seven planets huddle extremely close to their star, all within 1 fifth of Mercury's orbit. That makes a year on a Trappist-1 planet very short, from a day and a half on the innermost Trappist-1B to three weeks on Trappist-1H. That's a lot of birthday cake. The planets live so close together that they all tug on each other gravitationally. It's enough to slightly vary the lengths of their years. Their gravitational influence on each other allows us to figure out their masses, between 0.4 and 1.4 times Earth's mass.

This, combined with the depth of the eclipses, which gives their sizes, we can know their densities and so their compositions. They are very likely rocky or watery worlds, just like our own. The planets have now settled into stable orbital resonances with each other. And that's a hint that they may have formed further out and then migrated inwards. It's also unlikely that enough material resources would have been available for so much planet forming so close to the star. Because different chemicals condense at different temperatures, planets' distance from the star during formation largely determines the planet's chemical makeup. If they did form further out, then the Trappist-1 septuplets may have a lot of water and other volatiles. Maybe the Trappist-1 planets started out as mixtures of rock and ice. However, three of them now occupy their star's habitable zone, where planet surface temperature would be just right for liquid water.

We can estimate the location of the habitable zone for a given star based on the intensity of its photon flux and the effect of atmospheric greenhouse gases. Our solar system's habitable zone extends from roughly one astronomical unit, so Earth's orbit, to 1 and 1 half AU, so covering Earth and Mars. The same calculation for Trappist-1 forms a smaller, thin band. But in that densely packed system, it covers three of the planets. Yilon and team have done some climate modeling, which indicate that the three innermost planets may be more Venusian, so overheated due to a runaway greenhouse effect. However, that's really hard to predict at this point. The outermost planet, which orbits past the snow line, should be icy. However, it may be that internal heat left over from formation and generated by tidal interactions with its star and the other planets could warm it to liquid water temperatures as well. In fact, any of the seven planets could host liquid water. Should we pack our bags and move to Trappist-1?

What would it be like there?

We've already established that the temperature could be to our liking. Vines' Law tells us that the 2500 Kelvin Trappist-1 star shines brightest at infrared wavelengths. It would look pink on the sky to us.